Illumise. It wasn't so much the battle part that was tough, but the actual finding of the pokemon, as illumise is practically nonexistant in Sapphire version. When I first saw one in a trainer battle, I thought the pokemon was adorable, and decided it must be mine. After being unlucky with finding one, I eventually went on a rampage in route 117, destroying everything that showed up until I could get the pokemon I wanted, and even allowed one of my own pokemon to get defeated in this scourge. When I first found one, I somehow let it get away, causing my rage to steep even further. And then, when at last I caught one, with much rejoicing, I tried out my most precious catch... only to decide the pokemon's fighting ability didn't exactly measure up to my battle style, leaving me to bench the pokemon in my PC for eternity. ...Yeah, pretty lame end to the story.